1. Field of the Invention
The present invention relates to image forming apparatuses such as copying machines, printers, facsimiles, and multi-function peripherals (MFP) with a developing unit that develops an electrostatic latent image on an image carrier using a magnetic brush formed on the surface of a developer carrier.
2. Description of the Related Art
Two-component developers having a magnetic carrier and a toner are being used widely in image forming apparatuses that employ an electrophotographic method. At the time of developing electrostatic charge images, a magnetic brush is formed on the surface of a developer carrier, and a toner image is formed by making this magnetic brush come into contact with and rub against an image carrier.
In recent years, high image quality and high durability are being desired, and, in order to achieve these, measures are being taken such as reducing of the diameter of carrier particles and of the diameter of toner particles in a developing agent, or increasing the thickness of a coated plastic film covering the surface of the carrier particles made of ferrite, and the like, in order to give them charging characteristics. However, if these measures are taken, there is the problem that the amount of charge on the toner particles becomes high thereby reducing the developing characteristics. In view of this, in order to fully utilize the small diameter carrier particles and small diameter toner particles with a high resistance that reduces developing characteristics, it is desirable to use a counter development method with increased development efficiency in which the supply capacity of the developer is increased by conveying the image carrier and the developer carrier in mutually opposite directions at the closest part between them.
When high resistance carrier particles are used in the counter development method, a new problem occurs of “back edge density unevenness in outline characters”.
FIG. 8 is a diagram for explaining back edge density unevenness in outline characters.
Outline character back edge density unevenness is an abnormality of reduction in the density because of the density unevenness of the edge part of the bland image positioned at the back edge part of the outline characters formed inside the bland image.
As a proposed countermeasure for outline character back edge density unevenness, a developing unit disclosed in Unexamined Japanese Patent Application Publication No. 2002-72690 (hereinafter referred to as JPA 2002-72690), uses a two-component developing agent, a developing sleeve rotates in a direction opposite to the direction of rotation of a photoreceptor drum, and a bias voltage having superimposed AC voltage and DC voltage is applied to the developing sleeve at the time of development.
In this JPA 2002-72690, in order to provide an image with no sweeping line unevenness in the counter development method, a magnetic field of the carrier/magnetic field intensity and the position of a main development magnetic pole, particle diameter of the carrier, and the toner density have been stipulated.
However, in the developing unit disclosed in JPA 2002-72690, the improvement against outline character back edge density unevenness is insufficient, and in particular, it was found to become a problem when high quality images or outlines are required.
As has been explained above, when high resistance carriers are used in the counter development method, a new problem occurs of “back edge density unevenness in outline characters”.
The purpose of the present invention is to further improve outline character back edge density unevenness and to obtain good quality images.